<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style type="text/css">
        div {
            background: red;
        }

        p {
            float: left;
            width: 100px;
            height: 100px;
            background: green;
        }

        .c1 {
            clear: both;
        }
    </style>
</head>
<body>
<div>
    <p></p>
    <div class="c1"></div>
</div>
</body>
</html>